1. Technical Field
The present disclosure relates to control apparatuses for controlling a DC-DC converter having a switching element, and more particularly to a control apparatus for controlling a DC-DC converter in which the duty ratio of the switching element is controlled to convert an input voltage value of the converter to a predetermined output voltage value.
2. Description of the Related Art
In this type of DC-DC converter, the voltage applied between an input terminal and an output terminal of the switching element depends on the duty ratio of the switching element and the input voltage of the DC-DC converter.
As a control apparatus for controlling the DC-DC converter, for example, Japanese Patent Application Laid-Open number 2006-101680 discloses a control apparatus including a detecting circuit that detects an input voltage of the DC-DC converter in an on-vehicle high voltage system which is a primary side of the DC-DC converter. The control apparatus outputs a signal responding to the detected voltage value to a control device of the DC-DC converter in an on-vehicle low voltage system. The signal responding to the detected voltage value is obtained by a well-known PWM (pulse width modulation) processing of which the carrier frequency is changed depending on cases when detecting excessive high voltage or detecting voltage within normal range.
As a result, in the low voltage system having the control device that controls switching elements included in the DC-DC converter, when the carrier frequency corresponds to a case when detecting excessive high voltage, the control apparatus stops operating the DC-DC converter since reliability of the switching elements may degrade due to a voltage being applied between the input terminal and the output terminal of the switching element when the input voltage is excessively high.
Also, in the low voltage system, the control apparatus controls the duty ratio of the switching elements to be restricted in response to the voltage level of the PWM signal. In particular, when the input voltage obtained by decoding the PWM signal is excessively high, the DC-DC converter is controlled to be turned OFF.
It is noted that the high voltage system and the low voltage system are electrically isolated, and the low voltage system is connected to the vehicle-body.
The inventors have found that common mode noise increases when a failure such as an insulation failure occurs between on-vehicle high voltage system such as motor generator and the vehicle-body so that the noise is superimposed on the signal which is PWM processed. In this case, the input voltage obtained by decoding the PWM signal becomes excessively high so that the DC-DC converter may be stopped even when the DC-DC converter can be driven.